It is very useful for people to have their schedules open. I generally look at the FO's schedule I'm flying with, to get an idea of what they've been doing. Do they fly this trip all the time, have they just gotten off of long term sick, have they just checked out in the airplane from the 727 (uh oh), have they just had two months off. This information may not be volunteered by the other pilot.
Plus for trip trading purposes, it is very helpful. I'm not even going to ask a guy to swap if it's not a better deal for him/her. If I have no idea what else they are doing that month and if this trade would be workable for them, I'll just move to someone else and not waste my time. You suspect that people are hiding their schedules because they are doing something funky. Like calling in sick twice a month, every month. Working every day of their life (yes, people don't want to admit that). And on the MD-11, magically getting charters every single month.
There are some people that don't release their schedule OR their phone number. Now THAT is extreme jerkiness. I've been known to email someone, saying, "I was going to ask to bump you, but I can't view your schedule and I can't call you, so I asked someone else." That usually gets a quick reply.